Private key material should not be kept in the clear anywhere, ideally. This includes on your dev machine, serialised in a store, in the heap of your process, anywhere. Of course, it depends on your threat environment, but the article did mention pci-dss. If you put it in redis, then anyone that has access (internal baddies exist too!) can steal the key and sign something. Its hard to repudiate that.

How far do you go, how do you use the private key to sign something if you can't keep it anywhere?